Communications Satellites are used to transmit information from an uplink earth station to a transponder on the satellite and back down to a downlink earth station. Satellite transponders operate with various overall bandwidths up to about 72 MHz or more at the present time. There are a limited number of transponders on any given satellite (e.g., 40 or more) and given the cost of building and orbiting a satellite, lease or ownership of such transponders is expensive. When one satellite user cannot use the entire bandwidth of a leased transponder, it is desirable to allow others to do so. Present technology allows a plurality of users to share a transponder; however this is done by manually placing radio frequency (RF) carriers at various points in the transponder passband. These carriers are characterized by their center frequencies within the passband of the transponder and their bandwidths and represent the carrier for the modulated (usually digital) signal that a user will transmit over the transponder as a datalink. Present technology requires that a user manually set up his transmitting equipment so that the carrier is located at a specific center frequency of the transponder passband with a given fixed bandwidth. Accordingly, all his corresponding receiving equipment must likewise be manually set up to receive at that precise frequency. Because this manual approach requires that a carrier be, in effect, taken down when moved or adjusted, the manual approach requires a disruption in the flow of data over the datalink. This approach is not flexible, is disruptive and results in waste of expensive resources.
FIG. 1 is a diagram of an exemplar passband 100 (horizontal axis is frequency and vertical axis is signal magnitude) of a prior art communications satellite. As can be seen, only about 50% of the passband is actually used for the transmission of information, while much of it remains unused (i.e., those areas where no modulation is present such as area 102). While portions of unused bandwidth may not be utilized because no requirement for additional bandwidth exists, there are scenarios where a carrier having a bandwidth larger than any unused portion of bandwidth is required, thus giving rise to the need to consolidate and move existing carriers to the extent possible so as to create a space for such a larger bandwidth carrier. Given that the cost of building and placing a communications satellite into orbit is relatively fixed it would be desirable to use as much of the available transponder passband as possible.